In providing communications services to customers, service providers attempt to ensure that services are delivered without loss of data and with minimal interruption. A well known approach to ensuring data transfer services is automatic protection switching or APS. In SONET/SDH, APS 1+1 is typically used for single chassis protection switching.
Referring now to FIG. 1A, known single chassis APS 1+1 is discussed. A near end (NE) chassis 10 having SONET line-terminating equipment (LTE) whose data traffic is to be protected, has an NE working port 21 which is coupled via a bi-directional working link 14 to a far end (FE) working port 23 of an FE chassis 20 having SONET LTE. The NE chassis 10 is also coupled from an NE protection port 25 over a bi-directional protection link 16 to an FE protection port 27 of the FE chassis 20. In this configuration, the NE chassis 10 is said to be protected by an APS group having a working circuit made up of the NE working port 21, the working link 14, and the FE working port 23, and having a protection circuit made up of the NE protection port 25, the protection link 16, and the FE protection port 27.
Typically the working circuit carries the data traffic which is to be protected. When a circuit is carrying the data traffic, it is said to be active, and when it is not carrying the traffic it is said to be inactive. For consistency the link and ports of an active circuit are referred to as being active, and the link and ports of an inactive circuit are referred to as being inactive. In automatic protection switching the working circuit is typically active when there is no failure.
In the event of a failure or degradation of the signal of the active circuit, which may be caused by failure or degradation of the active link or either active ports, APS 1+1 switches the data traffic from traversing the failed or degraded circuit to traversing the other circuit. The other circuit becomes active and the failed or degraded circuit becomes the inactive circuit. Since each single chassis has control of a working port and a protection port, it is not difficult to switch the data traffic from the working circuit to the protection circuit.
The APS 1+1 architecture also allows for the protection circuit and the working circuit to be configured to end at two different FE chassis. Such a known configuration protects against nodal or router failures in addition to link and circuit failures.
Referring to FIG. 1B, a known dual chassis APS 1+1 configuration is discussed. A near end (NE) chassis 110 having SONET line-terminating equipment (LTE) whose data traffic is to be protected, has an NE working port 153 which is coupled via a bi-directional working link 114 to a far end (FE) working port 157 of a first FE chassis 120 (labeled “CHASSIS A” in FIG. 1B) having SONET LTE. The NE chassis 110 is also coupled from an NE protection port 155 over a bi-directional protection link 116 to an FE protection port 159 of a second FE chassis 130 (labeled “CHASSIS B” in FIG. 1B) having SONET LTE. The first and second FE chassis 120, 130 are coupled together via control link 140.
In this configuration, the NE chassis is protected by the APS group having a working circuit made up of the NE working port 153, the working link 114, and the FE working port 157, and having a protection circuit made up of the NE protection port 155, the protection link 116, and the FE protection port 159.
The second FE chassis 130 is referred to as the protection chassis or chassis in protection mode, and it is in constant communication with the first FE chassis 120 which is referred to as the working chassis or chassis in working mode.
In the event of a failure or degradation of the signal of the active circuit, which may be caused by failure or degradation of the active link, either active ports, or the working chassis, APS 1+1 switches the data traffic from traversing the failed or degraded circuit to traversing the other circuit. Since the FE chassis 120, 130 are remote from each other, FE chassis 120, 130 need to exchange switching control signals over the control link 140 to coordinate the switching from the working circuit to the protection circuit. FE chassis which exchange switching control signals over the control link 140 are said to be members of a redundant APS pair, each being an APS peer of the other within the pair. In switching the data traffic, the protection circuit becomes an active circuit and the failed or degraded working circuit becomes an inactive circuit.
Since the switching from the working circuit to the protection circuit is not local, communicating signals over the control link 140 to coordinate the switch takes time and there will be a delay before data transmission resumes on the new circuit causing an associated service interruption. The manner in which the switching from the working circuit to the protection circuit is carried out and the particulars of an APS control protocol used on the control link 140 are very important factors which determine the duration and magnitude of the service interruption.